The present invention relates to a method of forming a secondary emissive coating on a dynode, to a dynode coated by such a method, to a channel electron multiplier comprising a stack of such dynodes, and to an electron display tube, for example a cathode-ray tube or an image intensifier, including such a channel electron multiplier.
United Kingdom patent specifications Nos. 1,401,969, 1,402,549 and 1,434,053 describe different types of channel electron multipliers, each of which consist essentially of a stack of perforate, metal, electrically conductive layers. Each layer has a regular array of apertures with the apertures of each layer aligned with those of the other conductive layers in the stack so as to define the channels. Separators are disposed between each pair of adjacent conductive layers. The separators do not obstruct the channels. When the material making up the conductive layers is not sufficiently secondary emissive for a particular application, the secondary emissive properties of the conductive layers can be enhanced by providing a coating of a more emissive material at least on the exposed surfaces of the conductive layers inside the channels. This may be done on all of the conductive layers, but it may be preferable to apply the emissive coating only to the first few conductive layers located on the input side of the channel electron multiplier.
United Kingdom specification No. 1,523,730 describes dynodes suitable for use in channel electron multipliers. These dynodes consist of substrates bearing secondary emissive coatings of cermets of specified compositions, each containing an alkali metal fluoride. However, the secondary emissive coefficients of these cermets are not appreciably more than 4.
An article entitled "Growth of MgO Films With High Secondary Electron Emission on Al-Mg Alloys" by B. Goldstein and J. Dresner (Surface Science, Volume 71 Number 1, 1978, pages 15-26) discloses the formation of secondary emissive layers by the oxidation and activation of high purity sheet Al-Mg alloys having Mg-contents of from 0.1 to 3% by weight. The magnesium concentration in the surface oxide layers was increased by heating oxidized alloy sheets at temperatures on the order of 450.degree. C. Secondary emission coefficients (.delta.) of from 10 to 15 were obtained by this method. These alloys are not suitable for making dynodes for channel electron multipliers since an etching technique is not at present available for etching these alloys in the geometries desired for dynodes.
During the investigations which led to the present invention, it was found that secondary emissive coatings formed by evaporating aluminum-magnesium alloys containing from 0.1 to 3% by weight of magnesium could not be activated readily.